First thing you should learn to do is nothing. Dial back nutes, if dark or other observable problems. If you think there is a problem, the less you do the better until you know what it is. Only once you are certain should you act in any way that will impact your plant. Otherwise, you just as likely to throw kerosene on a dumpster fire.
Watering early depends a lot on pot size. If it is in an oversize pot for size of plant, try fertilizing where the roots are once a week, but rehydrate with pH'd water during the week as needed only. If soil or coco-like substrate, allow it to dry a bit before watering - less so for coco than soil. With soil, the top 1/2 to 1" should be dry before watering. Early on, you may want to water before 1" dries, but don't water everyday unless you are adding 10-15mL or something tiny.
The idea is for the nutrients not to build up while the plant lacks roots to suck it down. So, early on you don't need a weaker ppm or molarity, you just need to becareful of adding more when it isn't being used already. Rehydrating with ph-only or if you do feed make sure plenty runs out the bottom. That enusre the concentration does not skyrocket.
Small plant = small appetite. It doesn't mean it can't handle the same concentration, because it absolutely can, unless you are simply using too much regardless of plant size, or you are dumping more in while it is not being used up, which causes a continually rising concentration over time.
An ebb and flow of saturation is good for avoiding bad microbial growth in your substrate, too. If it is constantly moist, you will likely get some green algea on any perlite resting on top of soil, for example, or a much worse microbial problem.
Start gently lifting your pot to learn the weight of a saturated substrate and when it is ready for watering. A dry top layer is also a good indication you can water. Try to only water based on weight until you learn how the plant drinks. Soon, you'll be able to predict such things, but still be checking weight to verify.
If you ever see the plant wilt, you need to water a bit more per irrigation or less time between irrigations to avoid that wilt.
Relax. They probably aren't stunted unless you are seeing major growth issues -- if light is too close, the nodes and leaves will stack nearly on top of each other and look weird and toxicities are easy to see from leaf charts you can find. Seedlings need strong light from the get-go. Within 7 days it'll be 18-24" away or closer -- not sure on your lights. Heat is one concern. The other is too many photons for your plant. You'll see it cower and stop praying under conditions of too much light for too long.
Early on, seedlings don't always look the prettiest. If it is growing each day, slowly the first 2 weeks, then it is likely fine. Even if you see some discoloration or some imperfect leaves it is fine as long as it coincides with growth early on.
I have a seedling that's 1 week old. It has two sets of real leaves (not counting the tiny cotyledeons). I have another that is 14days old and it's third set of real leaves are now overlapping first set of real leaves and a new nodes is barely popping out. Growth nearly doubles each week and takes time to start rolling.
You likely only have to water every 2-3 days and upto 7 days in some cases, initially, but it depends on a lot of factors. Based on how most people are their first grow, you need to stay away as much as possible. Get over any OCDand the neurotic impusles that you feel. Ty to be a minimalist and add layers of sophistication as you learn. Walk before you run and learn better and avoid more mistakes. Be reactive, not proactive.. .you don't have the baselines yet to do so. Give it a couple grows before jumping the gun.
Get away from liquid nutes and any product with cartoon graphics on the label. Find a 2 or 3-part fertilizer, like jack's or general hydro i think has a very similar setup and similar price too. You want to provide what is neccessary with a complete set of nutrients, or you end up spending 10x more on additives that shouldn't be needed with a competent fertilizer brand.
e.g. you shouldn't need "cal-mag." Calcium and magnesium should already be in your fertilizer at an appropriate molarity. If it isn't, then it is crap fertilizer. Coco coir can require some prep to avoid cal-mag deficiences and k-toxicities, that's a special instance that is ameliorated by soaking in a cal-mag and non-potassium solution before using the coco. fertilize as normal after that and no problems will occur.
